Devices are already known for measuring the dust content of a gas stream, which include a source emitting electromagnetic radiation, for example light, and a photosensitive receiver. In these known devices the radiation is emitted in a beam directed transversely with respect to the direction of the mean velocity of the gas stream.
In a first embodiment of these known devices, termed "absorption opacity measurement," the receiver is arranged opposite the source with respect to the gas stream. This system has the disadvantage, inter alia, that the measuring signal is weak in the dust content range which is most interesting.
In another type of device of this kind, the receiver is on the same side as the source with respect to the gas stream but at a certain distance from the source, and this type provides measurement of flux scattered sideways by the dust. It is found in this type of device that the measurement signal increases with dust content until it reaches a maximum value, i.e. the saturation value for a certain dust content value. If the dust content increases beyond this value the measurement signal no longer increases but rather decreases, becoming weaker when the dust content is very high.
These two types of known devices also have in common the disadvantage of being applicable only to a small gas stream volume, whatever its dust content.